docs: def secure(pdf, user_pw, owner_pw, restrict_permission=True, pdftk=get_pdftk_path(), output=None):
      """
      Encrypt a PDF file and restrict permissions to print only.
  
      Utilizes pdftk command line tool.
  
      :param pdf: Path to PDF file
      :param user_pw: Password to open and view
      :param owner_pw: Password to transform permissions
      :param restrict_permission: Restrict permissions to print only
      :param pdftk: Path to pdftk binary
      :param output: Output path
      :return: Output path
      """
      if pdftk:
          # Check that PDF file is encrypted
          with open(pdf, 'rb') as f:
              reader = PdfFileReader(f)
              if reader.isEncrypted:
                  print('PDF is already encrypted')
                  return pdf
  
          # Create output filename if not already set
          if not output:
              output = add_suffix(pdf, 'secured')
  
          # Replace spaces within paths with backslashes followed by a space
          pdf_en = pdf.replace(' ', '\ ')
          output_en = output.replace(' ', '\ ')
  
          # Concatenate bash command
          command = pdftk + ' ' + pdf_en + ' output ' + output_en + ' owner_pw ' + owner_pw + ' user_pw ' + user_pw
  
          # Append string to command if printing is allowed
          if restrict_permission:
              command += ' allow printing'
  
          # Execute command
          os.system(command)
          print('Secured PDF saved to...', output)
          return output
      else:
          print('Unable to locate pdftk binary')